NEW MESOARCHAEAN DETRITAL DIAMONDS TEST SLAB-STACKING MODEL FOR ARCHAEAN CRATONIC KEEL FORMATION

Earth’s ancient tracts of continental crust – the cratons – owe their continued preservation to the formation of thick keels of buoyant, cool lithospheric mantle. These cratonic roots stabilised early continental crust, perhaps driving continental emergence, and contain valuable records of the tectonic modes that operated in deep time. When and how these cratonic keels were generated, and under what tectonic regimes is hotly debated. Detrital diamonds in Mesoarchaean-aged cratonic cover sediments provide a unique perspective on this issue, representing the only direct samples from these early cratonic keels. Recently discovered alluvial diamonds in the ~2.83 Ga Central Slave Cover Group sediments of the Tree River region in the Slave Craton, NW Canada, provide exclusive insights into the Mesoarchaean geothermal gradient, the mechanisms of lithospheric thickening, and the origin of diamond-forming fluids¹. However, previous studies were limited to three microdiamonds, only one of which contained appreciable nitrogen. Here we report the new discovery of 25 detrital diamonds recovered from the same lithologies within the ~2.85 Ga Tree River cover group sequence. Fourier-transform infrared spectroscopy (FTIR) indicates that five of these diamonds contain nitrogen at up to ~780ppm, with aggregation states ranging from type 1aA to 1aB. We will present carbon and nitrogen isotopic analysis to establish whether this larger sample set also contains the ¹³C-enriched δ¹³C and positive δ¹⁵N values previously reported for the Tree River diamonds, characteristic of a crustal-sourced diamond-forming fluid. We will discuss these results within the context of viable tectonic and thermal models for ancient cratonic mantle formation. This larger sample suit provides a unique opportunity to test various models of Mesoarchaean cratonic keel formation previously proposed for the Slave and other Archaean cratons.

¹ S. Timmerman, J.R. Reimink, A. Vezinet, F. Nestola, K. Kublik, A. Banas, T. Stachel, R.A. Stern, Y. Luo, C. Sarkar, A. Ielpi, C.A. Currie, C. Mircea, V. Jackson, D.G. Pearson (2022). Mesoarchean diamonds formed in thickened lithosphere, caused by slab-stacking, Earth and Planetary Science Letters, Volume 592, 117633, ISSN 0012-821X, https://doi.org/10.1016/j.epsl.2022.117633.